1. Field of the Invention
The present invention relates generally to wireless communication systems. More particularly, the present invention relates to a wireless communication system having an adaptive multi-rate (AMR) vocoder to maximize the voice quality while minimizing the level of channel coding.
2. Description of the Related Art
As the use of wireless communication systems become increasingly popular, a variety of methods are being developed to increase the number of mobile communication devices a system can simultaneously service. The Global System for Mobile Communications (GSM), also referred to as the Group Speciale Mobile, is one example of a wireless communication system which is constantly being adapted to increase the number of simultaneous users.
The GSM system is modeled after standards created by the European Telecommunications Standards Institute (ETSI) and operates between a telecommunication base station (BS) and a mobile station (MS) using a pair of frequency bands in a frequency division duplex (FDD) configuration. The first frequency band occupies the frequency spectrum between 890 to 915 Megahertz (MHZ), and the second frequency band occupies the frequency spectrum between 935 to 960 MHZ. Typically, the first frequency range is used for the lower power transmissions from the MS to the BS, and the second frequency range is used for the higher power transmission from the BS to the MS. Each frequency range is divided into 125 channels with 200 Kilohertz (kHz) spaced carrier frequencies.
The GSM communication system is a time-division-multiple-access (TDMA) system. In the GSM TDMA system, each carrier frequency is divided into eight (8) time slots. Because each MS is assigned a single time slot on one channel in both the first frequency range and the second frequency range, a total of 992 MS may use the BS at the same time.
A typical speech channel for GSM communication is sampled at 8 KHz and quantized to a resolution of 13 bits, providing for the digitization of speech ranging from 0-4 KHz by a voice encoder, also referred to as a vocoder. The 13 bits are then compressed by a factor of eight (8) in a full-rate vocoder to a voice data digital bit stream of 13 kilobits per second (Kbit/s). Because GSM uses a complex encryption technique with interleaving and convolution coding, a high-degree of system integrity and bit error control are achieved. In fact, despite multi-path and co-channel interference, the GSM system may continue to operate despite a carrier-to-interference ratio (C/I) as low as 9 dB, in comparison to a typical advanced mobile phone service (AMPS) analog system requiring a maximum C/I of 17 dB.
Depending upon the topography of an area, a typical BS may provide communication services to any number of MSs within a radius up to 35 Kilometers. Consequently, with the rising popularity of mobile communication devices, it is often the case that during peak periods of use, such as rush-hour traffic, all channels are fully occupied and the BS is not able to provide communication services all of the MS in its region.
In order to avoid the inability to service all MS within a region, the ETSI has contemplated a modification of the GSM standard to increase the density of the communication channels. However, because the allocated frequency spectrum of 25 MHZ with 125 separate 200 KHz carrier channels is fixed, a current approach to increasing the density of the communication system is to increase the number of users per channel. In general, this density increase is achieved by decreasing the amount of digital information which is sent to and from each BS, thereby allowing each BS to support more users in a 200 kHz frequency band.
One approach to decreasing the amount of digital information passing between a BS and a MS is to decrease the vocoder rate of the digital voice data from a full-rate vocoder rate of 13 kilobits per second (Kbits/s) to a half-rate vocoder rate of 5.6 Kbits/s. Although the ability currently exists to effectively double the number of users on any one communication channel from eight (8) to sixteen (16) by using the half-rate vocoder, it has been found that the 5.6 Kbits/s vocoder rate is barely acceptable as the speech quality is significantly decreased.
In light of the above, it would be advantageous to provide a communication system that provides for the user density of a half-rate vocoder system, while providing the voice quality approaching or exceeding that of a full-rate vocoder system. It would also be advantageous to provide a communication system that provides for the modification of the communication channel to incorporate only the amount of channel coding necessary to achieve a reliable communication link between the MS and the BS.
Broadly, the present invention provides for a wireless communication system having the ability to increase or decrease the vocoder rate and channel coding in response to the level of interference present on the wireless communication channel, resulting in a communication channel having the best possible speech quality. This may be accomplished in either a full-rate or half-rate GSM communication system by decreasing the amount of channel coding during periods of low channel interference to allow transmission of more speech information, representing a higher vocoder rate and resulting in a higher speech quality. During periods of higher channel interference, the amount of channel coding may be increased to the maximum channel coding allowed in a GSM communication network. This increased channel coding provides for consistent and reliable call handling, and results in a lower vocoder rate having a lower speech quality.
In an embodiment of the present invention, a tire-division-multiple-access (TDMA) communication system includes a base station (BS) and at least one mobile station (MS), each transmitting and receiving an analog radio-frequency signal carrying digitally coded speech. The speech is digitally encoded using a vocoder which samples a voice signal at different encoding rates. Alternatively, the speech may be encoded using a number of different vocoders simultaneously, with each vocoder having a different encoding rate. During periods when the radio-frequency channel is experiencing high levels of channel noise or interference, the encoded voice channel having a lower encoding rate is chosen. This lower-rate encoded voice is combined with the high. degree of channel coding necessary to ensure reliable transmission. When the radio-frequency channel is experiencing low levels of channel interference, less channel coding is necessary and the vocoder having a higher encoding rate is used. The high-rate encoded voice is combined with the lower degree of channel coding necessary to ensure reliable transmission. The appropriate level of channel coding necessary for reliable transmission is determined by various channel metrics, such as frame erase rate and bit error rate.
The determination of the appropriate vocoder rate and level of channel coding for both the uplink and downlink may be determined centrally at the base station, with the vocoder rate and level of channel coding for the uplink being relayed to the mobile station. Alternatively, the appropriate vocoder rate and level of channel coding for the downlink may be determined by the mobile station, and the appropriate vocoder rate and level of channel coding for the uplink may be determined by the base station.